The present invention relates to optical scanners of the field of view of a scene observed by an infrared thermo-ographic system. Such systems generally include a first optomechanical device having an entrance objective, which focuses the infrared radiation on a plane of infrared detector(s) by means of a scanner comprising at least a slow raster scanning element that permits analysis of the scene by successive lines; and a second optomechanical device comprising a periscope-head mirror, tilted with respect to the optical axis of the entrance objective and able to turn about this axis, and a derotator turning at a rate one-half that of the head mirror. The system further includes electronic equipment for transforming the output signal from the detector(s) into a visible image of the field of view.
The present invention concerns such an optical scanner including an entrance objective, a reflecting mirror able to oscillate about a first tilting axis orthogonal to the optical axis of the aforesaid entrance objective and realizing slow raster scanning when it oscillates, a field lens, a prismatic drum turning about an axis orthogonal to the axis of the reflecting mirror and enabling a rapid scan, and a set of lenses and mirror that focus the infrared radiation on a plane of infrared detectors. The invention also relates to an infrared thermographic system making use of these optical scanners.
Devices of this type are used in particular in landscape visual display equipment, the landscape being perceived by infrared radiation at a wavelength longer than one micron. This visual display can be produced day or night, being in general more useful at night when direct vision is greatly reduced, or impossible. The spectral bands of radiation detected by these devices correspond to transparent atmospheric windows, either 3 to 5.mu. or 8 to 12.mu.. This latter spectral band is particularly related to the invention because it is perfectly adapted to the imaging of bodies at ambient temperature, a black body at 300.degree. K. having its emission maximum in the vicinity of 10.mu.. Optical scanners for infrared thermographic system are described in Chapter 7, "Three-Dimensional Analysis Systems," Pages 219-248, in the book by G. Gaussorgues titled La thermographic infrarouge [Infrared Thermography], published by Technique et Documentation on Dec. 2, 1980.
It is known that infrared thermography cannot be used to convert received radiation into an electronic image on the scene, i.e., to use targets similar to photocathodes. This is because the received radiation is so lacking in energy that the electrons cannot be detached from the electrode, and that all the radiation can do is excite a photodiode, or a photoconductor. Thus it is impossible to obtain electronic scanning of an observable scene on a target, such as is the case with a television camera or a Vidicon tube. Optomechanical scanning thus is necessary to reconstruct the image of the scene and, as was the case for the first television cameras, this scanning consisting of systematic point by point scanning of the scene under observation, each point of the scene, termed an elementary field, ultimately being converted into a real image on the detector.
The present invention applies to scans of an approximately rectangular scanning field that consists of line scans and raster scans. In known conventional scanners, such as those previously discussed, slow raster scan is caused by a reflecting mirror placed behind the entrance objective moving at the rate of one image for each oscillation of the mirror about a first tilting axis. The line scan is caused by a rotating prismatic drum that can function in the transmission or in the reflection mode at the rate of one line for each rotation of the drum equal to 2.pi./m, m being the number of prism faces.
These scanners can have one, or preferably several detectors, so that several lines can be obtained simultaneously. The electrical signals from the output of the detector(s) are processed to reconstruct an image of the scene, on a television receiver, for example, in which scanning of the screen by the electronic beam is synchronized with, and identical to, the scanning of the real image of the scene on the infrared detector. This mode of reconstruction of the scene termed the first or imagery mode. Scanners used for the imagery mode generally are adapted to the standards used in television, which, for example can be 25 images per second with each image being composed of 230,000 dots in 575 useful lines of 400 dots each. The objectives used generally have a small aperture angle, of the order of only a few degrees. This small field of view is necessary when high resolution is desired in order to capture in sufficient quantity the radiation flux emitted by the distant objects it is desired to identify.
This constraint is not well suited to a sector or panoramic scanning mode similar to that used in radars. Infrared monitoring has many problems: panoramic surveillance of the sky, and of the horizon terrestrial or maritime; and surveillance in a given sector. Such infrared surveillance could make it possible to replace radars when they are electronically jammed, or when their transmissions in the operating mode creates the risk of detection. Under these conditions it is conceivable that the infrared scanner could be made to operate in the imagery mode by causing it to pivot about itself in a sector or panoramic scan. The engineering problem thus posed is that of causing the system to rotate while at the same time retaining the connections with its accessories. In an infrared scanner the cooling system is indispensable for keeping the detectors at a low temperature. Interconnection of the rotative scanner and its cooling system as well as the electronic portions of the device (which comprise in particular the image reconstruction cathode ray tube (CRT) present substantial difficulties. Keeping the cooling system fixed poses connection problems that are virtually insoluble, and linking the rotation of the CRT to the movement of the optics complicates and increases the weight of the system, something that is very prejudicial. Also there is the need for a rotating electric collector ring assembly between the detector(s) and the associated electronic equipment.
Another approach that does not require rotation of the detectors involves the placement of a periscope-head mirror in front of the system and rotating about the optical axis of this system as indicated on pages 230 and 321 of G. Gaussorgues' book, already cited. Because the rotation of the head mirror introduces what is called image dumping (rotation of the image), it is necessary to rectify this image with a derotator, such as a Wollaston prism, a Pechan prism, or a Rantsch derotator, for example. The derotator should have a value of its angular position about the axis of the beam that is half that of the head mirror, that is, when the head mirror is rotating the instantaneous rotation rate of the derotator must be half that of the head mirror. The imagery mode however is retained in this form of operation such that raster scan is effected by the reflecting mirror, and line scan by the prismatic drum, and as noted, the imagery mode is ill-adapted to panoramic scan.